The present invention relates to a gas sensor control device for controlling a nitrogen oxide gas sensor and a nitrogen oxide Concentration detection method. Hereinafter, the term “front” refers to a gas sensing side with respect to the axial direction of a gas sensor and the term “rear” refer to a side opposite to the front side.
Japanese Laid-Open Patent Publication No. 10-288595 discloses a nitrogen oxide concentration detection apparatus consisting of a nitrogen oxide sensor and a sensor control device. The nitrogen oxide sensor includes a sensor element provided with a first oxygen pumping cell, a second oxygen pumping cell and an oxygen concentration measurement cell to define a first measurement chamber in communication with the outside atmosphere via a first diffusion rate control member and a second measurement chamber in communication with the first measurement chamber via a second diffusion rate control member. Each of the first oxygen pumping cell, the second oxygen pumping cell and the oxygen concentration detection cell has an oxygen ion conducting solid electrolyte layer and a pair of porous electrodes. The sensor control device detects a current developed between the porous electrodes of the second oxygen pumping cell by the oxygen pumping action during the application of a constant voltage to the second oxygen pumping cell, while supplying a current between the porous electrodes of the first oxygen pumping cell so as to keep the oxygen concentration of gas under measurement flowing from the first measurement chamber at a given level by the oxygen pumping action. The sensor control device further corrects for an offset in the detected second oxygen pumping cell current based on the first oxygen pumping cell current, and then, determines the concentration of nitrogen oxide (NOx) in the gas under measurement according to the corrected second oxygen pumping cell current. In this way, the nitrogen oxide concentration detection apparatus detects the NOx concentration of the gas under measurement using both of the first and second oxygen pumping cell currents.
In general, the nitrogen oxide sensor has a catalytic action to control or prevent the decomposition of NOx at the electrode surface. There thus arises a problem that the zero point of the nitrogen oxide sensor (i.e. the output point indicating that the concentration of a specific gas component in the gas under measurement is substantially zero) shifts as the catalytic activity of the nitrogen oxide sensor deteriorates during long-term use. In order to address such a zero point shift problem, Japanese Patent No. 3589872 and Japanese Laid-Open Patent Publication No. 2001-133429 propose sensor calibration techniques of adjusting the zero point of the nitrogen oxide sensor for high long-term detection accuracy.